Improved method of removing the residual toner particles from a photoconductive surface

ABSTRACT

An improved method of producing xerographic copies which includes the steps of charging a photoconductor with an electrostatic charge, exposing the charged photoconductive surface to discharge portions of the charge in a configuration of image and nonimage areas corresponding to the copy to be reproduced, developing the photoconductive surface with developer material including carrier and electroscopic toner particles triboelectrically arranged and transferring the toner particles from the photoconductive surface to a backing material. The improvement comprises simultaneously cleaning residual toner images from the drum surface and reuse of these residual toner images during image development then transferring the toner particles as aforementioned, and then charging the residual toner particles with a corona having a polarity opposite to the charging polarity on the drum surface prior to drum exposure and recharge to convert positively charged residual toner particles to a negative charge thereby preparing the particles for cleaning by development in the development zone by the carrier.

United States Patent m1 3,628,950

[72] Inventor John F. Wirley Primary Examiner-George F. Lesmes Webster,N.Y. Assistant Examiner-M. B. Wittenberg [21] Appl. No. 883,704Attorneys-Melvin A. Klein, Norman E. Schrader and James [22] Filed Dec.10, 1969 J. Ralabate [45] Patented Dec. 21,1971. [73] Assignee XeroxCorporation Rochester, N.Y. ABSTRACT: An improved method of producingxerographic r r copies which includes the steps of charging aphotoconductor with an electrostatic charge, exposing the chargedphotoconductive surface to discharge portions of the charge in a con-[54] IMPROVED METHOD OF REMOVING THE figuration of image and nonimag eareas corresponding to the RESIDUAL TONER PARTICLES FROM A copy to bereproduced, developing the photoconductive sur- PiioTocoNDUcTlvE SURFACEface with developer material Including carrier and electro- 4 Chin, 4Drawing Figs scopic toner particles tnboelectncally arranged andtransferring the toner particles from the photoconductlve surface to a[52] US. Cl 96/1. R, b king materi L The improvement comprisessimultaneously 1 R, 1 17/175, l cleaning residual toner images from thedrum surface and reu e of the e residual toner images during imagedevelop. [50] Field Of Search 96/13, 1.4, m n! th transferring the tonerparticles 5 aforementioned, 1; 7/175; i5/1-5 and then charging theresidual toner particles with a corona References Cited having apolarity opposite to the charging polarity on the drum surface prior todrum exposure and recharge to convert posi- UNITED STATES PATENTS tivelycharged residual toner particles to a negative charge 3,375,806 4/1968Nost 118/637 thereby preparing the particles for cleaning by developmentin 3,444,369 5/1969 Malinaric 250/65 the development zone by thecarrier.

/5 l a? I 14 1 I4 I 7 /4 a t I 0 0. O0 039 t 'IIIIIIIIIIIIIIIA 7IIIIIIIIIA IMIROVED METHOD OF REMOVING TI-IE RESIDUAL TONER PARTICLESFROM A PHOTOCONDUCTIVE SURFACE This invention relates to improvedelectrostatic imaging and more specifically to an improvement over thedevelopment of electrostatic latent images and the removal of theresidual toner images from a support surface.

It is universally known that a commercially successful mode ofdevelopment employed in automatic xerographic apparatus described inWalkup US. Pat. No. 2,618,551 comprises a developer generally consistingof toner and a granular material called carrier, which by mixingtriboelectrically acquire charges of opposite polarity, isgravitationally cascaded over the xerographic plate carrying theelectrostatic latent image. Although carrier typically comprisesspherical particles, in various other systems, the carrier may be invarious forms and substances including flat platelets, cubical solids,synthetic and natural fibers, metallic filings, and other. In additionto the cascade development system, magnetic brush, liquid developer,fluidized bed, powder cloud and other development systems are wellknown.

A commercially successful mode of cleaning employed in automaticxerographic apparatus is described in US. Pat. Nos. 2,751,616 and2,832,977, wherein a brush with bristles which are soft and of suitabletriboelectric characteristics, and yet sufficiently firm to removeresidual toner particles from the xerographic plate, is used to whiskresidual toner images from the surfaces of the xerographic plate. Inaddition, webs or belts of soft fibrous materials or tacky materials,and other cleaning systems are known.

In spite of the successes that have been achieved in cleaning, the priorart solutions to the problems in the development and cleaning steps inthe xerographic process are not entirely satisfactory. For example,cleaning still typically requires bulky apparatus and a separate anddistinct cleaning station. Experience has shown that the greater thexerographic process, the greater danger of toner powder escapingthroughout the mechanism and dusting the operating apparatus. Manycleaning systems typically require more than one pass through thecleaning station, requiring more time for cleaning the xerographic plateand thereby making the clean ing step one of the limiting factors in theoperating time of the xerographic cycle. Also, typically development andcleaning must be performed at different areas of the xerographic plate,which requires more apparatus to ensure that the portion of thexerographic plate being used to reproduce the desired image is correctlyregistered at each of the xerographic statrons.

Experience in the art of photoconducto'rs has shown that the greater thenumber of passes necessary to clean or develop the surface of saidphotoconductor, the fewer the number of cycles through which saidphotoconductor or xerographic plate can be used with acceptable imagequality. The surface of the photoconductor is partially abraded bymultiple passes through development or cleaning steps, and scratches inthe surface of the plate may mechanically pick up toner particlesthereby darkening the background areas of desired images. In addition,increased numbers of passes through development or cleaning stationstend to increase toner consumption and to impair toner concentration inthe development system. Each of these effects contributes to reducedimage quality in the prior art systems.

Effects to solve the above problems have led to new and differentimaging systems, such as, the system for simultaneous development ofelectrostatic latent images and removal of residual toner images fromthe image support as described in copending application Ser. No.789,031, filed on Dec. 31, 1968 in the name of Volkers et al.Unfortunately, toner particles in the developer material are notuniformly charged to the same polarity such that complete removal of theresidual toner images can be accomplished by the development system. Asa result the residual images build up and deposit onto the copy sheet asunwanted background.

The present invention is intended to overcome the above disadvantagesand to be an improvement over the system of the aforementioned copendingapplication.

it is, therefore, an object of this invention to improve electrostaticimaging systems.

It is also an object of this invention to provide a method fordevelopment of latent images and removal of residual images on a supportsurface.

It is another object of this invention to provide a system for thesimultaneous development and cleaning of an electrostatic latent imagesupport surface or electrostatographic surface.

It is another object of this invention to provide a system for thesimultaneous development and cleaning of essentially the same area of anelectrostatographic surface such as xerographic plate.

lt is another object of this invention to provide a system for thesimultaneous development and cleaning at essentially the same area of axerographic plate at the same station in a xerographic apparatus.

It is another object of this invention to clean the surface of axerographic plate in a single cleaning pass.

' photoconductive insulating layer of the xerographic plate, or

any other exposed surface of the xerographic plate or support surface.

It is still another object of this invention to eliminate unwantedbackground in xerographic copy.

The foregoing objects and others are accomplished in accordance withthis invention, which for the substantially simultaneous removal ofresidual images comprising residual toner particles from the xerographicplate and development of electrostatic latent images on essentially thesame area of said plate. Essentially, the principle upon which thisinvention is founded is that by converting the polarity of the residualtoner particles to a negative polarity after transfer, the residualtoner particles remaining on the plate surface after transfer are nownegative will remain so for a period while the xerographic plate doesnot thereby enabling efficient development cleaning in the developmentzone.

For a better understanding of the invention as well as other objects andfurther features thereof, reference is made to the following detaileddisclosure which should be read in conjunction with the accompanyingdrawing in which:

FIGS. 1(a), 1(b), and 1(c) show side schematic views of the xerographicplate of the prior art at the time of development, transfer, andtransfer after extended use, respectively, and

FIG. 2 is a schematic side view of xerographic apparatus adapted forcontinuous and automatic operation to carry out the method of theinstant invention.

Referring now to FIGS. 1(a), 1(b), and 1(c) there is shown a typicalxerographic plate 10 including a photoconductive layer 11 and backinglayer 12 made out of suitable material and having electroscopic tonerparticles 14 thereon electrostatic image forrnation. Toner particles 14normally have a negative charge for development of the image areasdesignated by numeral 15. However, due to some toner particles having apositive charge is a tendency for these particles to deposit on theedges of the image areas where the greatest difference is in thepotential of the electrostatic latent image formed on the plate is foundto exist. When the toner particles are transferred to copy sheet, theresidual image remaining on the plate is predominantly the positiveparticles as seen in fig. 1(b). After repeated use 'of the plate, thesepositively charged particles build up as indicated in FIG. 1(c) anddeposit on the copy sheet as unwanted background unless removed. it hasnot been possible to remove these particles by development cleaning thecarrier beads in the development zone are also positively charged and donot scavenge them from the plate.

Referring now to FIG. 2 which discloses a xerographic apparatus showingthe steps used in the xerographic process, but embodying the improvedsystem of the present invention there is shown a drum 20 withphotoconductor layer adapted for rotation past processing stations. Asthe surface of the drum advances during a xerographic cycle, a coronadischarge device 22 initially said surface at a charging station. Thecharged surface then advances through an exposure station 23 where thelight and shadow image desired to be copied is projected onto thesurface of the drum 20.

The charged and exposed surface of the drum now bearing theelectrostatic latent image corresponding to the light and shadow imageprojected thereon, then advances into the com binationdevelopment-cleaning station 24 according to the present invention aswill be described more fully hereinafter. Development and cleaning arecarried out by a cascade of developer comprising toner and carrier whichdevelops electrostatic latent images and at the same time removesresidual images comprising toner particles, typically adhering to thesurface of the drum in essentially the same area as will become moreapparent.

The area of the surface just cleaned and developed then preferablyadvances through a corona generating device 25 which recharges the drumsurface in preparation for the transfer step. At the transfer station,the developed image is transferred to a sheet of suitable backingmaterial 28, such as, paper by another corona generating device 26. Thetransferred image is then passed through a fixing station on a belttransport .30 where a heating device 32 permanently afiixes thedeveloped image onto the backing material.

In accordance with the present invention, the drum surface from whichthe image now supported on the backing material was transferred,continues to advance through the cycle, but now supporting only theresidual image of toner particles remaining after the transfer stepwhich are to be removed from the development zone as will become moreapparent. Positioned after the transfer station is a dual negativecorotron 35 which is connected to a high level DC voltage supply 36 ofnegative polarity to impart a negative charge to the positively chargedresidual toner particles thereby converting all the image particles to anegative charge. The drum surface then passes a discharge lamp 37 whichserves to return the drum surface to its normal residual voltage but notthe residual toner particles which remain negative to the fact that theresidual toner particles have a much longer time constant than the drumsurface. When the drum surface passes the charging corotron 22, thephotoconductive surface is able to charge up to the required potentialbut the residual toner particles are not thereby enabling the particlesto remain negative and to be removed by carrier in the cascadingtwo-component developer. It will be appreciated that the drum surface iscleaned in the development zone and the residual toner particlesintermixed with the developer material and recharged in propertriboelectric relation with the carrier for reuse in the system. Thedrum is now ready for another cycle which repeats the aforementionedsteps.

It has been found that a negative preclean current ranging frommicroamperes to about 40 microamperes and preferably about 20microamperes performs well when a positive charge current of about 100microamperes is applied at the drum charging station. At these values,the drum surface charges to about 330 volts and the residual toner toabout 207 volts until the drum surface discharges to about 130 voltsleaving a greater negative charge on residual toner than on the drumsurface. As the drum passes the charging station a charge current of 100microamperes provides a 1000 volt charge on the drum surface for aproper exposure. if the preclean current is too high, then the drumcharging must be increased, and it has been determined that a 1000 voltsurface charge is desirable for a proper exposure and print out. itshould be noted that the preclean current is supplied from a variablesource of potential to accommodate varying conditions of operation.

ln the past the residual toner particles had to be physically removed ata separate cleaning station or if simultaneous development and cleaningwere attempted, copy quality would drop off rapidly in time. By thepresent invention, the xerographic reproduction system is more efficientin that the removal of the residual image and its subsequent reuse inthe system is effcctuatecl. it should be readily appreciated how thisinvention contributes to the overall quality of copies as well as theefficiency of the copier system.

it has also been found that the toner particle size can be a significantfactor. Toner particle size affects the efficiency of the electrostatictransfer of toner to latent electrostatic images and the transfer ofresidual toner from the xerographic plate back to the carrier. lt hasbeen found that both processes become more efiicient with larger tonerparticle sizes. At a given toner concentration, smaller toner particlestend to cover more of the surface of the carrier beads thereby leavingless free bead surface available for development-cleaning or scavenging.The smaller toner particles are also less susceptible to beingphysically knocked from the: plate surface. lt has therefore been foundadvantageous to use toners having a particle size distribution whichcontains minimal amounts of relatively small toner particles. Tonerparticles may be classified as to particle size in a classifier for finedry powders such as the Sharples K8 Super Classifier, manufactured bythe Sharples Company, 424 West Fourth Street, Bridgeport, Pa. in theSharples scale, toner particles are measured in microns. Toners withparticles of average size by number in the range of about l to about 20microns, with negligible numbers of particles of size less than 5microns, gives results preferred over those of average size in the rangeof about 4 to about 7 microns, with about 50 percent of the particles ofa size less than 5 microns. Toners in both of the above ranges givedevelopment-cleaning efficiencies which are preferred over thoseattainable with particles of average size in the range of about 2 toabout 3 microns, with about percent of the particles less than 5 micronsin diameter. The smaller toner particles will still perform thedevelopment-cleaning, although the build up of toner-film on theapparatus typically accelerated.

Another parameter is toner concentration in the developer mixture. Theconcentration of toner affects developmentcleaning primarily in thedevelopment part of the process. The cleaning will go on, but if thetoner concentration is too high, the cleaned residual images will beredeveloped as quickly a they are cleaned. Hence, the limitingconcentration at one end is development capability (Le. sufficient tonerto develop electrostatic latent images) while the other end point is thelimit of the cleaning ability of the system. These concentrationlimitations depend on the degree of quality to be copy desired. Tonerconcentration is conveniently expressed in terms of mass per unitsurface area, said surface being the surface of the carrier particles orbeads. The advantageous cascade development-cleaning system of thepresent invention produces satisfactory results in toner concentrationranges of about Oil to about 0.4- mg. of toner per sq, cm. of carriersur-- face. At toner concentrations lower than about 0.1 mg/sq. cm.,development in extended unexposed areas of the image pattern stilloccurs, but image tone uniformity tends to fall off rapidly. At highertoner concentrations the ability to clean is reduced. The reduction incleaning capability may in part be: due to increases in the amount ofresidual toner retained as the residual image. lt is also thought thatthere is increased redevelopment of the residual image at these highertoner concentrations. A preferred range of toner concentration in thedeveloper mixture is about 0.2 to about 0.3 mg./sq. cm, Theseconcentrations indicate that it is most desirable to closely control thetoner concentration, preferably by automatic means.

Problems related to higher toner concentration include toner impactionand toner agglomeration, which may greatly reduce image quality.

it has been found that the addition of small amounts of dry solidhydrophobic lubricants effectively controls toner impaction andagglomeration. Such lubricants include metallic salts of fatty acidssuch as zinc searate, and other materials such as colloidal pyrogenicsilica particles such as Cab-O-Sil, available from the CabotCorporation, or various mixtures of such materials, An extensive groupof such lubricants is recited in copending application Ser. No. 702,306,filed Feb. 2, 1968. A preferred range of concentration for the lubricantis in the range of about 0.1 to about 1 percent by weight of toner.

The other component in the developer is a granular material calledcarrier" which by mixing with the toner particles triboelectricallyacquires charge of polarity opposite that acquired by the toner. Carriergranules may be any shaped solid particle from flat platelets to cubesto spherical beads. The carrier may be made of any suitable materialsuch as glass, plastic, metal or other granular material. Carriergranules of average size in the range of about 30 to about 1000 micronsperform satisfactorily. A preferred range of carrier particles size isin the range of about l00 to about 600 microns.

The advantageous system of the present invention is useful in anyelectrostatographic process having an electrostatic latent image supportsurface. in the preferred process, xerography, the electrostatic latentimage support surface is the surface of a photoconductive insulatinglayer. Selenium in its amorphous is found to be a preferredphotoconductive insulating material for use in xerography because of itsextremely high quality image making capability, relatively high lightresponse, and capability to receive and retain charged areas atdifferent potentials and of different polarity. Any suitablephotoconductive insulating layer may similarly be used in the practiceof the invention. However, it is found that the invention systemperforms more satisfactorily if the electrostatic latent image supportsurface is quite smooth. Typical photoconductive insulating layersinclude; amorphous selenium, alloys of sulfur arsenic or tellurium withselenium, selenium doped with materials such as thallium, cadmiumsulfide, cadmium selenide, etc., particulate photoconductive materialssuch as zinc sulfide, zinc cadmium sulfide, French process zinc oxide,phthalocyanine, cadmium sulfide, cadmium selenids, zinc silicate,cadmium sulfoselenide, linear quinacridones, etc. dispersed in aninsulating materials include: belends, copolymer, terpolymers, etc. ofphotoconductors and nonphotoconductive materials which are eithercopolymerizable or miscible together to form solid solutions and organicphotoconductive materials of this type include: anthracene,polyvinylanthracene, anthraquinone, oxadiazole derivatives such as2,5-bis-(p-amino-phenyl-l 1,3,4-oxadiazole; 2-phenylbenzoxazole; andcharge transfer complexes made by complexing resins such aspolyvinylcarbazole, phenolaldehydes, expoxies, phenoxies,polycarbonates, etc., with Lewis acid such as tetrachlorophthalicanhydride; 2,4,7- trinitrofluorcnone metallic chlorides such as aluminumzinc or ferric chlorides; 4,4-bis (dirnethylamino) benzophenone;chloranil; picric acid; 1 ,3 ,S-trinitrobenzene; lcholoroanthraquinone;bromal; 4-nitrobenzaldehyde; 4- nitrophenol; acetic anhydride; maleicanhydride; boron trichloride; maleic acid, cinnamic acid; benzoic acid,tartaric acid; malonic acid and mixtures thereof.

In addition to the advantageous use of the inventive system forsimultaneously developing and cleaning an electrostatic latent imagesupport surface, it is clear that the system of the present inventionmay also be used as a separate cleaning system. Thus, a one-passcleaning system using developer as the functional cleaning medium, alsoshows that the advantageous development-cleaning system of the presentinvention can be used as both a development system and a cleaningsystem, in any two-cycle electrostatographic process. In such two-cycle,which cycle is solely for the purpose of removing residual toner imagesfrom the electrostatic latent image support surface. Unlike the dualstation system described in the preceding paragraph, the two-cyclesystem achieves all of the objects of the preferred system, except thatthe recycling may rnvolve slightly more comp icated mechanisms andelectrical circuits.

Although the description of the preferred embodiments of the inventivesystem has been primarily directed to the use of the inventive system ina xerographic process, it is appreciated and intended that theadvantageous system of the present invention be incorporated in anysuitable electrostatographic process.

Although specific components and proportions have been stated in theabove description of the preferred embodiments of thedevelopment-cleaning system, other suitable materials and variations inthe various steps in the system as listed herein, may be used withsatisfactory results and various degrees of quality. In addition, othermaterials and steps may be added to those used herein and variations maybe made in the process to synergize, enhance or otherwise modify the properties of the invention. For example, various photoconductive materialsmay be used in xerographic plates, and various photoconductorthicknesses may require somewhat different parameter settings forpreferred results.

it will be understood that various other changes in the details,materials, steps, and arrangements of parts which have been hereindescribed and illustrated in order to explain the nature of theinvention, will occur to and may be made by those skilled in the art,upon a reading of this disclosure, and such changes are intended to beincluded within the principle and scope of this invention.

What is claimed is:

1. ln a method of producing xerographic copies which include the stepsof charging a photoconductive surface with an electrostatic charge,exposing the charged photoconductive to discharge potions of the chargeand a configuration of image and nonimage areas corresponding to thecopy to reproduced, developing the photoconductive surface withdeveloper material including carrier and electroscopic toner particlesin triboelectric relation and then transferring the electroscopic tonerparticles onto a backing material, the improvement which comprisesapplying a high level corona charge onto the residual toner particlesremaining after transfer and the photoconductive surface, then directingillumination onto the photoconductive surface then applying a coronacharge of the photoconductive surface, then applying a corona chargeopposite in polarity from the first mentioned charge to obtain aselected charge on the photoconductive surface prior to exposure, thenexposing the photoconductive surface and residual toner particles todischarge portions of the surface in configuration of image and nonimageareas as aforementioned, and then advancing the photoconductive surfaceinto the development area for simultaneous development and cleaning ofthe residual toner particles by cascading developer material includingcarrier whereby the residual toner particles are intermixed with thedeveloper material for reuse in the development area.

2. A method according to claim 1 wherein said high-level corona chargeis applied from about a 20 microamperes current where charging thephotoconductive surface is applied from about a lOO microamperescurrent.

3. A method according to claim 1 wherein said high-level corona chargeis supplied from a variable DC source to obtain optimum performanceunder varying conditions.

4. The method according to claim 1 wherein said photoconductive surfaceis formed in the shape of a drum and rotated past processing stationsrepeating the aforementioned steps continuously to produce multiplecopies automatically and continuously.

2. A method according to claim 1 wherein said high-level corona chargeis applied from about a 20 microamperes current where charging thephotoconductive surface is applied from about a 100 microamperescurrent.
 3. A method according to claim 1 wherein said high-level coronacharge is supplied from a variable DC source to obtain optimumperformance under varying conditions.
 4. The method according to claim 1wherein said photoconductive surface is formed in the shape of a drumand rotated past processing stations repeating the aforementioned stepscontinuously to produce multiple copies automatically and continuously.